The invention described herein relates to method and means for reducing the vibration in various parts of machines and mechanical mechanisms and in the various materials being operated on by such machines or mechanisms.
The present invention has broad application to the reduction of vibration generally, as will be apparent from the following disclosure, but the principles involved in its operation will be illustrated first in its application to the reduction of vibration in a circular saw blade for the primary purpose of reducing the amount of noise radiated, but also for improving the smoothness and accuracy of the cut produced by the blade. Other applications and the various procedures and considerations necessary for adapting the present design concept to such other applications are described.
Circular saws, such as table saws, radial arm saws and skill saws for cutting wood, plastic, or metal vibrate during their operation and create high noise levels that contribute to the permanent heating loss of the operator and other personnel in the vicinity of such operation. This is a problem of worldwide importance.
Federal regulations issued by the U.S. Occupational Safety and Health Administration limiting the noise exposure of industrial workers using such machinery, generally cannot be met by existing noise control methods. A long history of development of devices and means for controlling noise of circular saws shows little advance on the problem.
The steel needed for proper cutting generally has little internal vibrational damping and therefore the impacts between the saw teeth and the workpiece excite the saw blade and cause it to vibrate at high amplitude in its various resonant modes. The greatest excitations are experienced at the blade resonant frequencies that correspond to the cutting impact frequency or particular harmonics of the cutting impact frequency. At these frequencies, energy is readily accepted by the blade from the cutting impacts, and this energy is not readily dissipated by either the internal damping of the blade material nor by the wiping contact between the blade and the work piece and therefore much of the energy received is stored in the blade in the form of a vibratory motion that alternately exchanges the energy from a kinetic energy of lateral motion (perpendicular to the rotary motion in the plane of the saw blade) to a potential energy of bending of the blade from its normal flat, plane configuration. The bending may be made up of various combinations of patterns involving radial and circumferential nodes and antinodes. Some forms of resonant vibration involve vibrations in which the nodes and antinodes do not remain stationary with respect to the blade, but may progress around the blade at various speeds, depending upon the relation between the natural stationary resonant frequency of a particular mode and the frequencies of the driving forces arising primarily from the saw tooth impacts.
Each resonant vibrational mode builds up in amplitude until the rate of dissipation of energy from that mode (which, during each vibratory cycle, involves the loss of a very small fraction of the stored energy) just equals the rate at which energy is supplied by the driving forces. Little energy is dissipated by the internal damping of the blade material itself as is easily demonstrated by impacting the free blade with a small hammer or its equivalent and listening to the sustained ringing of the blade. The sharp character of the blade ringing will generally mellow as the ringing continues because the higher frequencies are more rapidly damped than the lower frequencies even when they all experience the same damping factor (i.e., the same fractional loss of energy per vibrational cycle) and the lower frequencies usually persist the longest.
If the air around the saw blade were removed, the resonant ringing would be sustained even longer because some energy is extracted in the form of sound waves that radiate from the blade surface. This radiated sound from the blade surfaces is the principal cause of excessive noise exposure of the operator and other persons in the vicinity of such a saw. It should be noted that, although the noise radiated is loud and creates a hazardous working environment, the energy drawn from the saw blade by the radiating sound is very small compared with the energy fed to the saw blade by the mechanical interaction between the saw blade and the work piece.
The smallness of the energy radiated by sound can be appreciated when one observes a saw blade freely rotating without contact with the workpiece; at some speeds, simply the interaction of the rotating blade and the air can cause the blade to vibrate so violently as to produce a painful, piercing whine. Thus it is clear that neither the internal damping of the blade and associated solid structure nor the damping of the surrounding air extract energy rapidly enough to limit the vibrational amplitude to a satisfactory value, even when the driving forces are as small as those arising only from turbulent interaction between the blade and surrounding air.
Many attempts have been made to add vibrational damping by changing the saw blade itself. Changes in composition of the blade material help a little, but large changes in damping are not possible with conventional steels. Composite blades have been made using (1) inserts of metals with high internal damping, (2) laminated surface plates on one or both sides of the blade fastened with a viscoelelastic binder, (3) laminating two equal steel blades together, using a similar binder, (4) attaching a viscoelastic outer layer on one or both sides of the blade. All of these have serious drawbacks. Inserts such as, for example, those described in U.S. Pat. No. 2,563,559, Sneva, Aug. 7, 1951, are expensive and, if not securely bonded, they may be dislodged and become hazardous missiles by accidental movement of the workpiece and, if not very intimately bonded to the main blade material, the inserts will be of little effect in extracting energy. Viscoelastic bonding materials tend to be thrown out by centrifugal force from high speed blades. External damping material suffers from this same restriction, but also limits the depth of cut of the saw to that distance that remains between the outer edge of the damping material and the outer tips of the teeth. Whatever portion of the blade extends beyond the damping material will have little coupling to the damping material and therefore may still vibrate with high amplitude and radiate noise.
Means used to contact the side of the blade have produced some damping. A broom handle pressed against a blade, for example, will reduce the noise output of a large blade by damping some vibrations, but this procedure is not very effective because it is highly selective and can couple with only a limited number of vibrational modes, leaving many if not most of the modes undamped and vibrating at large amplitude; some of these modes were present in the undamped blade and some are new ones created by the restraint of the contacting member. The contacting member, in addition to drawing energy from some vibrational modes, also draws energy from the rotation of the saw by its frictional drag and therefore adds to the power required from the driving motor and results in undesirable heating of the blade.
The qualified success of the broom handle type of experiments in effecting some noise reduction and, more particularly, reducing some of the gross vibrations in a saw, has contributed impetus to the design of sawblade stabilizing devices that do not have the frictional drag nor the heating problems produced by a physically contacting stabilizer. Examples of such efforts are disclosed in U.S. Pat. No. 3,540,334, McLauchlan, Nov. 17, 1970, and in U.S. Pat. No. 3,674,065, Fairfield, July 4, 1972. Both of these patents employ relatively small stabilizing presser plates, alternatively called vibration dampers, urged against the saw blade by a suitable force and held away from it by a fluid or combination of fluids allowed to escape at the interface between the plate and the blade from a supply reservoir at a high pressure.
The advantages claimed for the various forms of these devices are primarily increased accuracy of cut, improved smoothness of the cut edges, and reduction of the size of the Kerf or slot cut by the saw through the use of thinner blades, permitted by the reduction of the tendency of thin blades to vibrate when the damping devices are used. In one instance, U.S. Pat. No. 3,674,065, Col 4, line 33, the vibration control improved the accuracy of cut to 0.007 inch with the damper, from 0.050 inch without the damper. In all instances the improvement in performance was in the order of thousandths of inches and applies to the gross vibrations of the saw at low frequencies (related to the blade rotation frequency) which lie in the range of 10 to 100 Hertz (or cycles per second) for blades rotating in the region of 600 to 6000 rpm (revolutions per minute).
Dampers following the teachings of these designs do little in the way of controlling noise (airborne sound) radiated from the sawblade for a number of reasons. First, the dampers are small relative to the surface area of the saw-blade and can therefore couple with and control (or reduce the amplitude of) only a limited number of the numerous high frequency vibrational modes characteristic of the type of movement which radiates sound. Even when a number of dampers are used, as is proposed for some applications, the saw blade is restrained only at a very limited number of points and the remainder of the saw can still vibrate in modes which have little or no active motion at the locations of the dampers so they continue actively to radiate sound when the dampers are applied. Second, the vibrations associated with acoustic noise (airborne sound radiation) have very much smaller amplitudes than those for which the vibration dampers of the prior art are designed to control. The acoustic freqencies of interest lie generally in the region of 500 to 10,000 Hertz. Third, the damper designs illustrated by the prior art teach the use of a recessed plenum of a minimum of 0.001 in. to aid the distribution of the fluid used as the bearing medium between the saw and the damper. Any space used as a plenum reduces the effectiveness of the damper for converting vibrational energy into air flow which in turn is used to convert that energy into heat. Fourth, the edge of each damper is constructed as a narrow rim around the recessed plenum which follows the convention generally used for the design of fluid bearings; however, that design does not work well toward converting vibrational energy into heat. It tends to restrict the flow of air across the rim and confine the air in the recessed plenum where it is simply compressed as a spring and is then effective in pushing the saw blade away after the compression. The air thereby returns most of the energy of compression back to the saw. At acoustic frequencies the net effect is only to have decreased the amplitude of the saw blade by alterning its vibrational wave shape a miniscule amount as a result of the increased pressure against it as it progressed toward the damper. To obtain any substantial decrease in the vibration at acoustic frequencies, means must be provided for taking energy from the air before that energy can be returned to the saw during the next half cycle. None of the designs of prior art teach how to withdraw or to absorb energy from the air.
Vibrations of the saw blade in the acoustic frequency range of primary interest, 500 to 10,000 Hertz, are excited (or receive their energy) primarily from the tooth impacts rather than from the irregularities in the blade rotation. The acoustic frequencies that do most of the radiation are those frequencies characteristic of the tooth impact frequency or any of its harmonics that lie at or near any of the numerous natural resonant frequencies of the saw blade. The small amplitude of these vibrations can be appreciated by considering the situation for a 10 inch diameter wood-cutting circular saw for which the sound pressure measured close to a blade sawing a 3/4 inch pine board was in the order of 130 dB re 0.0002 microbar (millionths of an atmosphere).
Most of this sound energy was at 1000 Hz and above. The displacement amplitude of the saw blade necessary to generate this pressure, if all the energy were concentrated at 1000 Hz, would be under 0.001 inch; if the energy were distributed to higher frequency components the amplitude would be lower since the amplitude required to create a given pressure varies as 1/frequency.
The objective of the present invention is to reduce the radiated noise at the operator's position (approximately 2 ft. from the saw blade) to meet the requirements of the Occupational Safety and Health Administration or surpass them. That agency now requires the sound level to be below 90 dBA where personnel are exposed for periods of 8 hours daily and that requirement may be lowered to 85 dB in the foreseeable future.
Conventional noise reducing means have relied upon extracting vibrational energy at acoustic frequencies from the blade by employing the high internal losses in selected materials that undergo extensional or shearing deformation, and designing arrangements of such materials so as to optimize their deformation as a saw blade vibrates. This seems to be the most obvious approach because the application of damping materials is known to be highly effective on panels and other relatively thin vibrating members which in themselves have little internal damping. The difficulties previously mentioned notwithstanding, much effort has been expended in improving this means of energy extraction.
It is not obvious that the very much less effective process of extracting energy by acoustic means from the side of the blade without making any physical, solid contact to the blade could be made even more effective than the process of damping by adhesion of solid damping materials to the blade. However, a number of substantial advantages to such a process has encouraged its exploitation and the results have led to the extremely effective and beneficial noise control means set forth in this disclosure.
The advantages sought and achieved include:
(1) A high degree of reduction of radiated noise over the broad audible frequency region; PA1 (2) Freedom to utilize the full cutting depth normally available with an untreated saw blade; PA1 (3) Freedom to use any conventional saw blade with smooth sides; PA1 (4) Freedom to interchange sawblades at will as the workload may demand; PA1 (5) Avoidance of costly modifications or additions to conventional saw blades; PA1 (6) Increased life of saw blade due to reduced fatigue from vibration; PA1 (7) No need for decreasing saw blade strength or increasing its thickness as is necessary for some configurations using solid damping means; PA1 (8) Automatic application of damping when the saw is in use and withdrawal of the damping means for ease of access to the saw when the saw is deactivated; PA1 (9) Simultaneous stabilization of low frequency vibrations characteristically produced by rotational irregularities thereby to provide for more accurate cutting, for better surface finish, and for the use of thinner blades where thicker blades would have been required without the benefit of such stabilization.